The present invention relates to electric generator structures, and more particularly, to a piezoelectric sensor having a particular electrode arrangement.
The advancement, at lower cost, in the art of precision flight control and guidance of aircraft, missiles and space vehicles depends in part on progress in sensor technology. Present computer technology allows sophisticated and complex signal processing at reasonable cost, but the information processed is frequently derived from sensors having a cost which is a disproportionate part of the system cost. Other typical sensor limitations are poor accuracy and low reliability.
A characteristic inherent of piezoelectric devices that depend on remanent polarization is that they are susceptible to changes in output impedance and scale factor if exposed to substantial electrical, mechanical, or temperature stress. These changes affect the repeatability of the device parameters. The effects of offending stresses are time dependent, with long-term stresses being more detrimental than short-term stresses. Instrument applications such as accelerometers can be implemented without exposing the piezoelectric device to large DC voltages; therefore, electrical stress can be virtually eliminated as a factor affecting repeatability. Mechanical stresses occur at interfaces between the piezoelectrical material and other material of the device due primarily to mismatches in temperature coefficients of expansion. In addition to interface mechanical stress, temperature changes can cause device parameter changes; however, much of the change appears to be reversible and thus has minimal effect on scale-factor repeatability. The net effect of temperature swings is thus mechanical stresses exerted on the active piezoelectric material at the various interfaces associated with the device. Such temperature-induced mechanical stresses result in thermal hysteresis effects and changes of scale factor; therefore, for instrument applications, undesired or uncontrolled interface stresses must be reduced to the greatest extent possible.
Instruments utilizing commercially available piezoelectric ceramic sensors are susceptible to appreciable scale factor uncertainty, when exposed to extreme temperature variations such as those encountered in aerospace environments. Commercially available piezoelectric ceramic sensors are constructed, for example, of two layers of lead zirconate titante (PZT) bonded to a central layer of brass or steel. Representative ones of such devices were tested in the bender mode and were found to be unstable when exposed to repeated extreme temperature cycles; a scale factor change of five percent after several cycles was common. The tested devices exhibited long-term as well as short term changes. Accordingly, there is a need for a piezoelectric sensor having long-term stability and repeatablity of output impedance and scale factor in an environment of extreme variations of temperature.